The present invention relates to a transmission system, for a repeater monitor signal, using an optical fiber amplifier as a repeater in which a rare earth element such as Er is doped and, more particularly, to a monitor control signal receiving apparatus, for an optical fiber amplifier, for receiving a monitor signal superposed by performing amplitude modulation for a main signal.
In a general optical amplifier, signal light is propagated together with pump light through a rare-earth-element-doped optical fiber obtained by doping a rare earth element such as Er in an optical fiber, thereby amplifying the signal light. FIG. 2 shows a conventional monitor control signal receiving apparatus for an optical fiber amplifier. Referring to FIG. 2, reference symbol c denotes an input signal; and d, an output signal. Reference numeral 21 denotes an optical signal input terminal to which a received optical signal is applied. A main signal amplitude-modulated by a monitor control signal is input to the optical signal input terminal 21. This received optical signal is input to an Er-doped optical fiber 23 through a wavelength division multiplexing coupler 22. On the other hand, similarly, pump light output from a pumping semiconductor laser diode 31 is input to the Er-doped optical fiber 23 through the wavelength division multiplexing coupler 22.
The received optical signal input from the optical signal input terminal 21 of the Er-doped optical fiber 23 is pumped by the pump light input from the pumping semiconductor laser diode 31 to be optically amplified. The optical signal amplified by the Er-doped optical fiber 23 is transmitted to an optical signal output terminal 24, and is partially monitored by a pin photodiode 25 and a current/voltage converter 26. A monitor control signal extractor 32 extracts a monitor control signal from the monitored waveform, and transmits a reproduced monitor control signal to a monitor control signal output terminal 33.
A monitor voltage output from the current/voltage converter 26 is compared with a reference voltage 27 by an operational amplifier 28, and a low-pass filter 29 sufficiently removes a monitor control signal component from the monitor voltage. Thereafter, the voltage is fed back to a pumping laser diode driver 30. The operational amplifier 28 controls the laser diode driver 30 such that the average level of an optical signal output is kept constant.
In this conventional monitor control signal receiving apparatus, since a monitor control signal is extracted from an output signal from the optical fiber amplifier, a monitor control signal which is not necessary for an optical signal receiver cannot be suppressed by an optical fiber amplifier controller. For this reason, the characteristics of the optical signal receiver may be adversely affected by the monitor control signal.